Arrange a wire rack over a baking sheet. Combine the butter with the herbs. Loosen the skin on the turkey breasts and place the butter between the skin and breast meat, working out to the edges evenly. Place a bay leaf between the skin and meat. Spray the skin with olive oil, and sprinkle liberally with salt and pepper. Roast to 165 degrees internal temperature, about 1 hour. Remove and cover loosely with foil to cool back to room temp, reserving the drippings for the gravy. Then cover and store for a make-ahead meal, as well as another night's meal, such as the Turkey Club Enchilada Casserole.
For the stuffing:
Melt 6 tablespoons butter in large skillet over medium heat. Grease a casserole dish with 1 tablespoon of remaining butter, and reserve the rest to dot the top of the stuffing once it is in its casserole dish. To the melted butter, add the herbs, celery, apples, onions, bay leaf and some salt and pepper. Cook to tender, 15 to 20 minutes, stirring occasionally. Add the stuffing cubes and moisten with the stock until damp, but not wet. Fill the casserole dish and dot the top with butter. Cover and refrigerate to store.
For the gravy:
Melt the butter in a large skillet and season liberally with black pepper. Whisk in the flour until light golden in color. Then whisk in the stock and Worcestershire and simmer at low boil to thicken. Add any pan drippings from the roasted turkey breasts. Once the gravy lightly coats the back of a spoon, add a ladle of it to the egg yolk while beating with fork. Then add the yolk back to gravy to gloss it up, stir in. Add the mustard, if using. Add salt to taste. Cool and store.
Preheat the oven to 375 degrees F. Place the rack in center of oven.
Bring the stuffing to room temp, then bake until crispy on top and hot through.
Reheat the gravy over medium heat, partially covered and stirring occasionally.
While the stuffing is in the oven and the gravy is reheating, heat up an additional couple cups of plain stock in a high-sided skillet. Thinly slice 1 turkey breast, removing the bay leaf when you get to it.
For each sandwich, very lightly toast a slice of bread and place on a dinner plate. Top the bread with a scoop of stuffing (I use an ice cream scoop). Set a few slices of turkey into broth to heat through. Arrange the turkey slices over the mound of stuffing. Liberally ladle the gravy over the top of the sandwich and garnish with chopped parsley. Save any leftover gravy for another meal, such as the Turkey Club Enchilada Casserole later in the week.